Latest developments in nanofluid flow and heat transfer between parallel surfaces: A critical review

Highlights

• Nanofluid flow and heat transfer between parallel surfaces are studied.

• Parallel plates, parallel disks, and concentric pipes are considered.

• Physical phenomena and influential parameters are evaluated.

Abstract

The enhancement of heat transfer between parallel surfaces, including parallel plates, parallel disks, and two concentric pipes, is vital because of their wide applications ranging from lubrication systems to water purification processes. Various techniques can be utilized to enhance heat transfer in such systems. Adding nanoparticles to the conventional working fluids is an effective solution that could remarkably enhance the heat transfer rate. No published review article focuses on the recent advances in nanofluid flow between parallel surfaces; therefore, the present paper aims to review the latest experimental and numerical studies on the flow and heat transfer of nanofluids (mixtures of nanoparticles and conventional working fluids) in such configurations. For the performance analysis of thermal systems composed of parallel surfaces and operating with nanofluids, it is necessary to know the physical phenomena and parameters that influence the flow and heat transfer characteristics in these systems. Significant results obtained from this review indicate that, in most cases, the heat transfer rate between parallel surfaces is enhanced with an increase in the Rayleigh number, the Reynolds number, the magnetic number, and Brownian motion. On the other hand, an increase in thermophoresis parameter, as well as flow parameters, including the Eckert number, buoyancy ratio, Hartmann number, and Lewis number, leads to heat transfer rate reduction.

Introduction

Fluids play a significant role in numerous industrial processes as heat carriers. The flow and heat transfer characteristics of fluids in thermal systems, ranging from car radiators to nuclear reactors, determine the performance of such systems. The performance of a thermal system can be improved through the enhancement of heat transfer. Heat transfer enhancement can result in more compact thermal equipment which saves energy and space and minimizes cost (Fig. 1). Various approaches classified as active and passive are widely used to improve heat transfer in many applications (Fig. 2). “Active” corresponds to the cases where an external power (force) is used for heat transfer enhancement, such as applying a magnetic field or creating vibration on the surface of a thermal device. On the other hand, “passive” represents techniques in which there is no external driver, such as extended surfaces (fins) and turbulators, for the increase of heat transfer. Using nanofluids instead of conventional liquids in thermal systems is also a passive technique for heat transfer improvement.

One of the major limitations in heat transfer applications is the low thermal conductivity of conventional working liquids such as water, ethylene glycol (EG), and engine oil (EO). In recent years, nanofluids with a superior thermal conductivity compared to those of these base liquids have been introduced for improving the heat transfer performance in various applications [[1], [2], [3], [4], [5], [6]]. Solid nanoparticles with various atomic structures have been used as additives to base liquids. The selection of nanoparticle type depends on the particular application as well as economic considerations.

In the literature, there are a number of review articles on nanofluids. For example, Ganvir et al. [7], Solangi et al. [8], and Suganthi and Rajan [9] summarized the studies performed on the thermophysical properties of nanofluids. Lomascolo et al. [10] presented a review of the recent experimental research on the conductive, convective, and radiative heat transfer in nanofluids. Considering the heat transfer in nanofluid flows, Hussein et al. [11] concentrated on the improvement of forced convection heat transfer using nanofluids, Haddad et al. [12] summarized the research on the natural convection heat transfer of nanofluids, and Pinto and Fiorelli [13] discussed the mechanisms responsible for improved heat transfer. In addition, Mahian et al. [14,15] reviewed recent developments in simulation methods and applications of nanofluid flows and discussed the challenges in the field.

There are also some reviews on modeling and simulation of nanofluid flows and heat transfer using different CFD approaches [16], numerical and semi-analytical approaches [17], and traditional and novel approaches such as single-phase effective media, Lattice Boltzmann methods, Eulerian-Lagrangian methods, mixture models, and thermal dispersion [18].

The most important thermophysical properties of nanofluids are thermal conductivity and dynamic viscosity [[19], [20], [21], [22], [23]]. The increased thermal conductivity of a nanofluid enhances the heat transfer rate, and the increase of dynamic viscosity intensifies the friction of fluid, thus demanding a higher pumping power. In this regard, a series of investigations were performed on the improvement of the thermal conductivity of nanofluids. Younes et al. [24] summarized the recent publications regarding the dispersion of nanoparticles to the base fluid to augment the thermal conductivity of nanofluids. Tawfik [25] presented a summary of experimental studies performed on the improvement of thermal conductivity of nanofluids and discussed the effects of nanoparticle size, shape, and concentration, as well as base fluid type and temperature. Aybar et al. [26] described the mechanisms of thermal conductivity enhancement of nanofluids and summarized the models proposed in the literature. Regarding the rheological characteristics of nanofluids [27], the relevant experimental measurements [28] and the proposed dynamic viscosity models [29] were described. Azmi et al. [30] presented a comprehensive review of the research progress on improvement of the effective dynamic viscosity and effective thermal conductivity of nanofluids.

The hydrothermal performance of nanofluids in different industrial applications has also been reviewed. Kasaeian et al. [31] reviewed the recent application of nanofluids in porous media for thermal system applications. They described the effects of porosity, inertial coefficient, and permeability of porous media along with the influences of nanofluid parameters on the flow and heat transfer characteristics. Mahian et al. [32] and Kasaeian et al. [33] reviewed the influence of using nanofluids on the performance of solar water heaters, solar collectors, solar stills, solar cells, and thermal energy storage from environmental and economic points of view. In addition, they described the difficulties in utilizing nanofluids in solar energy systems. The application of nanofluids in heat exchangers [34] has been studied along with their behavior subject to magnetic fields in [35,36]. Amani et al. [37] presented a review of the influence of nanoparticles on the mass transfer performance and hydrodynamic behavior of liquid–liquid systems.

In recent years, many reviews of forced convection of nanofluid flows in tubes or natural convection in square and rectangular cavities have been published. However, only a few reviews in the literature were concerned with the evaluation of the flow and heat transfer of nanofluids in annuli. Togun et al. [38] reviewed the studies conducted on the heat transfer of fluids and nanofluids in various annular passages, including triangular, square, rectangular, elliptical, and circular configurations. Dawood et al. [39] summarized the investigations performed on the heat transfer analysis of fluids and nanofluids in concentric and eccentric annuli. Ahmed et al. [40] provided an overview of the thermal characteristics as well as applications of fluids and nanofluids in annuli.

Despite the extensive literature cited above, there is no review article that focuses on the thermal performance of nanofluid flow between parallel surfaces. Thus, in this paper, a comprehensive review is conducted on the hydrodynamic and thermal characteristics of nanofluids between parallel surfaces. The flow between parallel surfaces can be found in various types of heat exchangers that are used in different industries. The flow between parallel surfaces are classified into three groups as follows:

• Nanofluid flow between parallel plates/disks. Heat and fluid flow between two parallel plates/disks occur in many engineering applications such as cooling towers, micro-sized cooling systems, food processing, chemical processing equipment, lubrication systems, polymer processing, preparation, fog dispersion, and hydro-dynamical machines [40].

• Nanofluid squeezing film flow between parallel plates/disks. Recently, squeezing film flows between parallel plates/disks have been used extensively in engineering applications such as lubrication systems (e.g., load-bearing systems and flow of oil in bearings), compression and injection shaping, and food and polymer industries [41].

• Nanofluid flow through an annulus. Flow in a concentric annulus appears in cooling systems (e.g., electrical gas-insulated transmission lines and thermal insulations), thermal storage systems, and heat exchangers [42].

Due to a large number of investigations on the flow and heat transfer of different types of nanofluids between parallel surfaces, in this review, various sections are devoted to summarizing the effects of nanoparticle concentration, particle migration, magnetic field application, nanoparticle shape or type, different dimensionless numbers, slip velocity, presence of suction or blowing, a heat source or heat sink, use of non-Newtonian nanofluids, porous media, thermal radiation, rotating systems, heat flux ratio, radius ratio, and inclination angle on the heat transfer behavior of nanofluids. Fig. 3 schematically illustrates the aim of the present study. The present review paper mainly concerns the recent advances in these areas.